1. Field of the Invention
The present invention relates to a method of modulating transmission data and demodulating received data. More particularly, the present invention relates to a method of modulating and demodulating data represented in two types, that is, high and low.
2. Description of the Related Art
Generally, a transmitting end of a communication system, which sends out signal to a receiving end, carries out certain predetermined processes to reduce error of signal transmission. Therefore, the transmitting end carries out modulation with respect to the transmission data, and the receiving end carriers out demodulating processes with respect to the received data to recover to the initial data.
FIG. 1 shows a general data (signal) transmission in a conventional communication system, and FIGS. 2A to 2H show the waveforms of the signal being processes in the transmitting and receiving nodes of the communication system. As follows, the process of modulating and demodulating data in a general communication system will be described in detail with reference to FIG. 1, and FIGS. 2A to 2H.
In FIG. 1, a communication system includes a transmitting node A, a transmitting node B, and a receiving node C. Other elements may be included in the communication system. However, FIG. 1 only shows the above-mentioned elements for easier understanding.
The transmitting node A generates data (a) to transmit to the receiving node C. The transmitting node B generates data (b) to transmit to a node other than the receiving node C.
When the data to transmit is ‘1’, the data is expressed as ‘−1’, and when the data for transmission is ‘0’, it is expressed as ‘1’. As illustrated in FIG. 2A, the data (a) is ‘101’, and according to FIG. 2C, the data (b) is ‘110’.
The transmitting node carries out modulation with respect to the data for transmission. Accordingly, the transmitting node spreads data for transmission by using orthogonal codes. By the orthogonal code expansion, error rate of the data in the transmission channel can be reduced, The transmitting node spreads the transmission data by using the orthogonal code as allocated to the receiving node. The orthogonal code may include Walsh code.
Accordingly, the transmitting node A spreads transmission data by using the orthogonal code which is allocated to the receiving node C. In FIG. 1, the receiving node C is allocated with the orthogonal code ‘w0’. With reference to FIG. 2B, the code ‘w0’ is ‘0101’, and through data expansion as illustrated in FIG. 2E, the code ‘w0’ is spread to ‘1010 0101 1010’. The transmitting node A sends out the spread data over the antenna.
The transmitting node B spreads transmission data by using the orthogonal code which is allocated to the receiving node. With reference to FIG. 1, the orthogonal code ‘w1’ is allocated to the receiving node. With reference to FIG. 2D, the code ‘w1’ is ‘0011’, and through the data expansion as shown in FIG. 2F, the code ‘w1’ is spread to ‘1100 1100 0011’. The transmitting node B sends out expansion data over the antenna.
The receiving node C receives the spread data from the transmitting node A and from the transmitting node B. Accordingly, the receiving node C needs to extract data which is transmitted from the transmitting node A. The process by the receiving node C of extracting the data of the transmitting node A, will now be described below.
FIG. 2G shows the data received at the receiving node C. With reference to FIG. 2G, the receiving node C receives summation data of the data of the transmitting node A and the transmitting node B. For the convenience of explanation, the receiving node C therefore receives data of ‘−2 0 0 2 0 −2 2 0 0 2 −2 0’.
The receiving node C reverse-spreads the received data with the orthogonal code it is allocated. More specifically, the receiving node has allocated with the orthogonal code of ‘0101’, which is converted to ‘1 −1 1 −1’ for use in the modulation and demodulation process. Therefore, the receiving node reverse-spreads the received data ‘−2 0 0 2 0 −2 2 0 0 2 −2 0’ by using ‘1 −1 1 −1’.
With reference to FIG. 2H, the receiving node C obtains ‘−2 0 0 −2 0 2 2 0 0 −2 −2 0’ by carrying out the reverse-expansion.
The receiving node C segments the obtained data in the unit of orthogonal code length, and averages the segmented data. More specifically, the receiving code C obtains an average ‘−1’ with respect to ‘−2 0 0 −2’, obtains an average ‘1’ with respect to ‘0 2 2 0’, and obtains an average ‘−1’ with respect to ‘0 −2 −2 0’. As the receiving node C obtains ‘−1 1 −1’, the transmitting node A can obtains transmission data ‘101’.
However, because the transmission data is expressed in three types, that is, ‘−1’, ‘0 (no data)’, ‘1’, the data range for reception at the receiving nodes increases as the number of transmitting nodes increases. In other words, when there are five transmitting nodes, the receiving node needs to receive data of ‘−5’ to ‘5’. Accordingly, bits increase to receive the data, and subsequently load also increases to process the increased data.
Furthermore, the above-explained method is not suitable for a communication system which transmits data in two types, that is, high and low. Accordingly, a method of data modulation and demodulation, which can be used in a communication system that expresses data in high and low type, is required.